The IEEE 802.11a standard makes use of the Orthogonal Frequency Division Multiplex (OFDM) transmission scheme. The main feature of the OFDM scheme is that the information stream is not transmitted into a single carrier, but is divided into several sub-carriers, each transmitting at a much lower rate.
Furthermore, all these sub-carriers are orthogonal, i.e., they overlap their spectra, but do not cause mutual interference.
The fact that the different sub-carriers overlap their spectra makes one of the main operations at the receiver especially difficult: synchronization. In a receiver, the synchronizer is the block responsible for detecting the incoming frame and for estimating and correcting possible frequency offsets. The synchronization process is also responsible for providing a reference channel estimation to the channel estimation block. It further decides the starting point from which on the different OFDM symbols will be fed into the FFT block. The correct reception of OFDM signals is very sensitive to the synchronizer performance.
In the OFDM transmissions considered here, the information is not transmitted continuously, but in bursts. Each burst contains a single frame, which is a compound of different OFDM symbols.
In an OFDM transmission each data packet consists of a preamble and a data carrying part The preamble symbols are placed at the very beginning of each frame during transmission. The preamble consists of 10 “short” identical known OFDM symbols concatenated with 2 “long” identical and known OFDM symbols. The preamble symbols have a very specific periodic structure in the standard IEEE 802.11a to simplify synchronization. The data carrying part consists of a variable number of OFDM symbols, where each OFDM symbol contains useful information plus some known pilot sub-carriers, which are typically used for phase tracking.
Synchronization comprises the following operations: Frame detection, carrier frequency offset determination, symbol timing estimation, extraction of the reference channel, and data reordering. The synchronization process is data-aided, i.e., based on the digital processing of the preamble symbols.
During reception, the synchronizer has to peer at the channel in order to detect an incoming packet or frame. Frame detection in a receiver is an especially difficult task because there is no time raster that governs the transmission of the frames. In other words, the receiver does not know when to expect an incoming frame. The object of frame detection is to determine the symbol boundary so that correct samples for a frame can be taken. A major problem in the use of OFDM is therefore the determination of the time instant, at which the receiver starts sampling a new frame. A mismatch in the determination of this parameter would introduce a phase error causing intercarrier interference (ICI).
A method for frame detection in an OFDM signal is described by Chiu Ct al. (Yun, Chiu, Dejan Markovic, Haiyun Tang, Ning Zhang, OFDM receiver Design, published at URL: http://bwrc.eecs.berkeley.edu/People/ Grad_Students/dejan/ee225c/ofdm.pdf). It uses the first ten short symbols transmitted at the beginning of an OFDM frame, also referred to as the “short training sequence”. The waveform of the short symbols is known and stored in the receiver device. The receiver performs a correlation of the sampled signal with the stored waveform. It further performs and autocorrelation of the sampled signal with the delay of one short symbol. While the autocorrelation creates a first signal with a plateau over a time span during which short symbols are received, the correlation with the known waveform creates a second signal exhibiting peaks. The last peak of the second signal occurring during a plateau in the first signal is chosen as a time reference to start frame detection.
However, in the solution proposed by Chiu et al. the frame detection does not work if a fading channel is affecting the signal.
Another method is described in Schwoerer, L.; Wirz, H.; “VLSI Implementation of IEEE 802.11a Physical Layer”. Proceedings of the 6th International OFDM-Workshop (InOWo) 2001, pp. 28.1-28.4, September 2001, Hamburg, Germany. To detect the periodicity of the short training sequence, a sum of the absolute amount of three delayed autocorrelation terms over three different, overlapping time spans is calculated and then compared to a threshold value that is scaled with the signal power over the same time spans.
The solution proposed by Schwoerer et al. correspondingly makes use of three autocorrelators for the frame detection. This implies a rather large silicon area necessary for implementation of their solution and a high power consumption.